Our furnace is a two stage one that was installed last year (a Bryant 80t). When it was installed, it was installed with a thermostat for a one stage furnace. This appears to be because this model furnace has a mode where it will accept being controlled by a single stage thermostat and determine for itself when to fire the second stage based on its memory of recent cycles.

We just got one of those new fangled Nest thermostats that has the ability to control the second stage directly. When I installed the new thermostat, I left it configured as it was before (not wanting to change too many things at once), so the new one is controlling the furnace as a single device.

I'm curious now whether I could expect any significant difference in performance if I reconfigure the setup to allow the thermostat to control the second stage? Or, will I see similar results because the furnace probably is smart enough to activate itself at an appropriate time?

1 Answer
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Without the thermostat telling it, the furnace has to guess how much heat you need, and those guesses may not always be right. The thermostat will know how far you from your target temperature and know which stage you need to be using. That means a thermostat wired up for both stages will heat up faster when you get back from vacation, and the temperature stays more constant all other times.

This was what I was thinking. I'll give this a try and see how it goes.
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SredniVJan 24 '12 at 15:09

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This is a good answer. In my suite, the thermostats are set back to about 50F when we leave (there's an automation interlock with the intrusion detection system). When someone arrives back home, the HVAC controller calls for both stages of heat. This heats the suite at about 9F for every 7 minutes. With only one stage, it takes about 4x as long. So, with two stages of heat we can allow the temperature when unoccupied to drop further, thereby reducing waste heat while absent, and still return to a comfortable temperature in the same amount of time when you get home.
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alx9rJan 11 '13 at 23:26